Mold wall, especially a broad side wall of a continuous casting mold for steel

ABSTRACT

The invention relates to a mold wall for plate molds, tubular molds or similar, especially a broadside wall of a continuous casting mold for steel. The inventive plates consisting of copper or a copper alloy, which are either provided with coolant channels or are in thermally conductive contact with a water tank. Said plates have a surface which comes into direct contact with the steel melt and a protective layer applied to said surface. The wear resistance and mechanical workability are improved as a result of the protective layer consisting of a galvanically manufactured binary or ternary metal alloy dispersion, e.g. based on nickel with intercalated dispersants.

The present invention relates to a mold wall for plate molds, tubularmolds or the like, in particular, a broad side wall of a continuouscasting mold for steel including plates of copper or a copper alloywhich are either provided with coolant channels or are in a thermallyconductive contact with a water tank and which have a surface that comesin a direct contact with steel melt and a protective layer appliedthereto.

In order to increase wear resistance of the copper molds, such asCompact Strip Production mold plate, slab mold plates, tubular plates,and beam-blank molds, they are electroplated with chromium and/or nickeland, nowadays, also with nickel-cobalt alloys. These layers noticeablyincrease, due to their larger hardness and scaling resistance, the wearresistance of molds and, thus, lead to a noticeable increase of holdingtime.

Dependent on the use, these layers, which are applied to copper molds,have different thickness. The drawback of these layers consists in thatthey, because of their large hardness, can be mechanically treated onlywith much difficulty and, thus, their production is associated withcomparatively large costs.

As a result, on many occasions, a compromise is sought between the wearresistance and the economical finish-treatment of the layers.

Further, the nickel hardness falls at increased temperatures by about50%, and the hardness of nickel-cobalt and solid nickel by about 30%.

In the industry, e.g., in the production of racing motors or in toolproduction, since some time, nickel-silicon carbide-dispersion layersare used. Here, we deal with high wear resistant layers which at thesame time have a high thermal resistance.

Tests have shown that the microstructure of metals or metal alloys canbe, changed by inclusion of dispersing additives. In a plurality ofcases, the change leads to the increase of the wear resistance and thethermal resistance. It is known that besides the silicon carbideparticle, the inclusion of ultra diamonds also leads to the improvementof material characteristic, i.e., of wear resistance.

The grain size range of included dispersing additives varies from about10 to 1000 nanometers in many practical application. The experimentsalso showed that the material characteristics resulting from dispersionare influenced by the size of the dispersing additives. Because of this,dependent on the load, different sizes of the dispersing additives areused.

A still non-laid open, German application DE-100 18 504.5 discloses useof hardenable copper alloys for molds. The invention consists in the useof a hardenable copper alloy for mold, in particular for producing broadside plates for thin slab continuous casting molds, and containing from0.1% to 0.5% beryllium and from 0.5% to 2% nickel.

German Publication DE 26 34 633A1 discloses that in a continuous castingmold for casting steel and including a metal body provided with an innerlayer of a wear-resistant material, the wear-resistant layer consists ofelectrolytically or electrolessly deposited metal layer with particlesof a solid material, which does not dissolve in electrolytes, includedin a crystal lattice. At that, a wear-resistant nickel layer can containmetal carbide particles included in the nickel lattice. Further, as ametal carbide, silicon carbide and as solid material particles, adiamond dust can be used. The solid material particles can also consistof metal oxides. The binary nickel dispersion layers are available witha hardness from about 380 to 450 HV1 and a high wear resistance at bothroom temperature and at a temperature from 350 to 500° C.

German Publication DE 198 01 728 C1 discloses a continuous casting moldfor casting steel strands and consisting of mold plates and water tanks,which are connected with each other, between which water cooling iseffected by using water conducting channels arranged in a side of awater tank adjacent to a mold plate. The mold is characterized in thatthe mold broad side elements such as the copper plate and the water tankwith or without water conducting channels, but with a connection plateprovided with water conducting channels, are held with coupling boltswith conical heads which are received in substantially conical recessesformed in the copper plate, and are held together with a tightingelement.

Proceeding form this state of the art, the object of the invention is toso improve a mold plate for forming in particular, the broad side wallof a continuous casting mold for steel, with respect to its wearresistance at high temperatures upon contact with a steel melt and withrespect to its economical treatment, e.g., smoothing, that the servicelife of the mold plate is significant by prove in comparison with thestate of the art.

According to the invention, this object is achieved by forming theprotective layer of the mold wall of the type discussed above of binaryor ternary metal alloy dispersion produced by electroplating, on thebasis of nickel with inclusion of dispersing additive. These measuressignificantly increase the processability and the wear resistant ofso-called “hot face” of a mold wall.

Dependant on the load on the mold wall caused by steel grade,temperature, and/or turbulence of the melt in the mold, advantageously,materials such as cobalt, iron, zinc, copper, manganese, and chromiumare added to nickel by electroplating as alloy components.

According to advantageous embodiment of the invention, it iscontemplated to use as dispersing additives for further improvement ofmechanical and physical properties of the protective layer:

a) carbides of titanium, tantalum, tungsten, zirconium, borum, chromium,and silicium;

b) oxides of aluminum, chromium, silicium, beryllium, and zirconium.

A significant advantage of the invention results from the fact that,e.g., nickel-cobalt-silicon carbide dispersion layer has, at hightemperatures, e.g. in the range between 350 and 500° C., a much smallerreduction of the hardness than, e.g., super-clean nickel, nickel-cobalt,and hard nickel. The abrasion rate of nickel is 16 times higher than,e.g., abrasion rate of a binary nickel-cobalt-silicon carbide dispersionlayer with 380 to 450 HV1, although the dispersion layer is only twiceas hard as the super-clean nickel layer with 380 to 450 HV1 against 220HV1.

In comparison with a nickel-silicium dispersion layer, the abrasion rateof a binary nickel-cobalt-silicon carbide dispersion layer amounts onlyto about 10%.

The basis for this difference consists, on one hand, in silicon carbideparticles and, on the other hand, in the microstructure of thedispersion layers.

Despite the achieved high wear-resistance, the binary alloy dispersionlayers can be economically treated because in comparison, e.g., with ahard nickel alloy with 600 HV1 at the room temperature, they have ahardness in the range between 380 and 450 HV1, within which they stilleconomically treated, as experience has shown.

According to one embodiment of the invention, the binary or ternarynickel-alloy allotropes form a basis for, in particular, a multi-layerdispersion coating of mold plate inner surfaces.

A further embodiment of the invention is characterized in that both themechanical and physical properties of a dispersion layer such as wearresistance, and/or thermal stability, and/or tribology are adjusted bychanging the microstructure by varying the inclusion of nano sizeparticles, in particular, silicon carbide particles.

This provides the operator with a possibility to select optimalconditions with respect to wear characteristics and economical treatmentfor existing loading of a mold wall.

Preferably, dispersing additives with particles sizes from 1μ to 5μ ornanosize particles with a size from 10–100 nanometers are used. The sizeand the inclusion rate of dispersion additive is selected, e.g., inaccordance with tribological requirements.

According to a further embodiment of an inventive mold wall, forimprovement to a most possible extent of mechanical properties of theprotective layer, as dispersion additives, non-metallic solid materials,such as boron-nitride, boron-carbide, silicium-nitride, and ultradiamonds are used.

Finally, the mold wall according to the invention is characterized inthat the dispersion layers have a thickness from 10–10.000μ which variesdependent on the load during casting and a necessary subsequenttreatment.

The attached drawings show:

FIG. 1: a diagram illustrating hardness of Ni allotropes at roomtemperature or after a thermal treatment; and

FIG. 2: a diagram illustrating abrasion rate before and after a thermaltreatment.

The diagrams clarify big advantages of binary NiCo 30, nickel-cobaltsilicon-carbide-dispersion with hardness of about 450 HVI in comparisonwith:

Ni (super-clean nickel)

NiCo (nickel-cobalt alloy)

Ni (hard nickel)

Nip 12 (electrolytically produced nickel alloy with more than 12% ofphosphorus)

Nickel silicon carbide dispersion layers NiSiC with inclusion of 5% SiC

NiSiC dispersion with hardness of 360 HV1

NiSiC dispersion with hardness of 440 HV1

NiSiC dispersion form modified electrolytes with hardness of 420 HV.

1. A mold wall of a continuous casting mold for steel and having platesof copper or a copper alloy and which are either provided with coolantchannels or are in a thermally conductive contact with a water tank, themold wall comprising: a surface that comes in a direct contact withsteel; and a protective layer applied to the contact surface and havinga thickness of from 10μ to 10,000μ, wherein the protective layer isformed of a binary nickel-cobalt alloy NiCo30 having a hardness of about400 HV1 and includes dispersing additives with a particle of 1μ to 5μ ornanosize particles with a size of 10–1000 nanometers, wherein thedispersing additives are selected from a group containing: a) carbidesof titanium, tantalum, tungsten, zircomium, boron, chromium and silicon,and b) oxides of aluminum, chromium, silicium, beryllium, and zirconium,and wherein the dispersion layer has adjustable mechanical and physicalproperties such as wear resistance, and/or temperature resistance and/ortribology defined by a microstructure changeable by varying inclusion ofnanosize particles of NiCo 30.